1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and an image forming program, particularly to an image forming apparatus, an image forming method and an image forming program for forming an image by distribution of two or more types of dots having different densities per unit area in the same hue.
2. Background of the Invention
A printing apparatus for printing an image by a personal computer or the like is exemplified by a printer using the ink of three colors, magenta (M), cyan (C) and yellow (Y), and a printer using the ink of four colors composed of the aforementioned three colors plus black (K). Such a printer allows an image to be formed by turning on or off the dot of each hue.
In recent years, by contrast, there have appeared many types of printers wherein, in order to get a high-quality output image, inks of different densities within the same hue (e.g. dark colored ink and light colored ink) are used or different ink dot diameters are utilized so that a dot having a plurality of gradation levels in one hue is formed.
However, even if the aforementioned dots each having a plurality of gradation levels in one-hue can be formed, the image may deteriorate in some cases if these dots are not arranged adequately. For example, to get an output image having a large dot (having a large area, hence high dot density per unit area) and a small dot (having a small area, hence low dot density per unit area), binary halftone processing is applied to each dot; then the pixel values are added up. Thus, although the distribution of small dots alone (FIG. 11(a)) and distribution of large dots alone (FIG. 11(b)) are satisfactory, distribution of the overlapped dots cannot be said to be satisfactory in many cases, with the result that the graininess in the final image considerably deteriorate (FIG. 11(c)).
Solution to this problem has been found in the method wherein a plurality of types of dots having the different densities per unit area are arranged in the form associated with one another. For example, such a method is disclosed in Patent Document 1.
In Patent Document 1, dot positions for relatively high density dots per unit area are determined by dither method, dots positions for relatively low density dots per unit area are determined by error diffusion method, and when performing the error diffusion, by reflecting the quantization error caused by the processing of the relatively high density dots, good distributions of the two types of dots are obtained.
Incidentally, “two or more types of dots having different densities per unit area” includes in addition to the dots having different densities such as high density dot, mid density dot and low density dots, the dots having different dot sizes such as large dot, middle dot and small dot, and combinations of different density dots and different sized dots such as high density dot and small dot, or large dot and low density dot.
Patent Document 1: Official Gazette of Japanese Patent No. 3208777
Patent Document 2: Official Gazette of U.S. Pat. No. 6,264,300
Patent Document 3: Unexamined Japanese Patent Application Publication No. 2003-501300
However, in the aforementioned Patent Document 1, the reflection of the quantization error is restricted to unprocessed pixels, and processed pixels cannot be reflected with the quantization error. Sometimes, this causes the case where dots are not well dispersed.
The example of this case is shown in FIG. 12. FIG. 12 is a figure of the image formed according to Patent Document 1, where the high density dots are determined by dither method, and low density dots are processed by the error diffusion method reflected with the quantization error caused by the dither method. In this method the processing is performed with repetition of the scanning direction, in the figure, from right to left and left to light, while proceeding from up to down, this movement is so called a zigzag scanning. As can be seen from FIG. 12, in the unprocessed pixel (at underside of the high density dot or down stream of right/left scanning direction) dot generation is suppressed by the influence of the error from the high density dot. However regarding the low density dots generated previously to the high density dot generation, since the influence of the error can not be reflected, the low density dots are generated at the upside or upstream scanning side of the high density dots. This causes to worsen the dispersion of the total image and to deteriorate the image quality.
Therefore, a method for obtaining better dispersion has been required.